1. Field of the Invention
This invention relates to a miniature current-controlled actuator in which when a stator coil is energized, a rotor with a permanent magnet is rotated through a preset angle in a direction corresponding to a direction in which the stator coil is energized.
2. Description of Related Art
In the technical field of a camera, a miniature current-controlled actuator, sometimes referred to as a moving-magnet motor, is known. This actuator is constructed so that when a stator coil is energized, a rotor with a permanent magnet is rotated within the range of a preset angle in a direction corresponding to a direction in which the stator coil is energized, and an output pin actuated integrally with the permanent magnet drives a member to be driven. The actuator, in contrast with a stepping motor, has significant advantages of affording low cost, compactness, and small power consumption. Thus, in the camera, it is chiefly used as a driving source for shutter blades or stop blades, but its application is not limited to the camera and can be made to various products.
For such actuators, various structures have been proposed and used, and typical examples of the structures in recent years are set forth in Japanese Patent Kokai Nos. 2000-197326 and 2000-292827. The actuators described in these publications are such that although the structures of stators, as well as those of rotors, are different from each other, magnetic poles magnetized in the radial direction of individual permanent magnets are equal in number, and even though the rotors are replaced with each other, they will function properly, insofar as they gives rise to no shape or dimension problems. Although, for the rotor, one described in the former publication appears to be simpler in fabrication, it entails the high cost of material and is hard to obtain a great magnetic force (high magnetic flux density). Consequently, in most cases, one described in the latter publication is used as an actuator for cameras.
Here, the structure of the rotor of this type will be specifically explained. The permanent magnet magnetized in the radial direction has a cylindrical shape. A rotary shaft which lies in a hollow portion of the permanent magnet and whose ends projecting therefrom are supported by bearings of the stator and an output pin (driving pin) located in the radial direction are integrally constructed of synthetic resin. Such an output pin is often provided as a single one. Even with the use of the actuator in the camera, however, two output pins may be provided at symmetrical positions of 180°, depending on the structure of the shutter blades or the stop blades, and the number of pins can be chosen properly in accordance with a desired specification. Such a component part made of synthetic resin, after being fabricated as an independent member, may be attached to the permanent magnet by cementation or force fit. In view of cost, however, it is advantageous to attach the component part to the permanent magnet on molding the component part through a so-called outsert process of injection molding.
For this actuator also, the need for further compactness has recently been emphasized. However, in the actuator of the type set forth in the above Kokai No. 2000-292827, the outside diameter of a cylindrical stator has already been reduced to as small as 4-5 mm, and the permanent magnet of the rotor has also been reduced to as small as 2 mm in diameter. Thus, the situation is that further compactness is extremely difficult. In particular, in the permanent magnet of the rotor, even when such a small diameter is slightly reduced, the proportion of a reduction in mass is increased, and it becomes difficult to ensure a preset magnetic force or to positively obtain permanent magnets within tolerances for mass production. As such, when an attempt is made to achieve further compactness, the problem arises that cost is greatly increased.
It is thus conceivable that the permanent magnet is configured into a column shape so that even when the diameter is reduced, mass which remains unchanged can be ensured. When the permanent magnet is configured into a column shape without reducing the diameter, a great magnetic force is necessarily obtained. In such a case, however, a question arises as to how portions supported by the bearings and the output pin are constructed. In order to solve this, the structure that the portions supported by the bearings and the output pin are fabricated as independent parts of special shapes so that the permanent magnet is attached to them by cementation or force fit has been proposed. However, as parts diminish in size, it becomes difficult to set up the parts, one by one, by cementation or force fit and to positively produce good rotors, and actuators thus fabricated involve high cost as a matter of course.